As a sealing material, there is used a composite powder material comprising glass powder and a refractory filler. This sealing material is excellent in chemical durability and heat resistance and suitable for securing airtightness as compared to a resin-based adhesive.
Conventionally, PbO—B2O3-based glass has been used as the glass powder (see, for example, Patent Literature 1). However, from an environmental view, a demand for removal of PbO from a glass composition has led to development of Bi2O3—B2O3-based glass. Patent Literature 2 or the like discloses that the Bi2O3—B2O3-based glass has a low melting point and has chemical durability comparable to that of the PbO—B2O3-based glass.
There can be achieved a decrease in thermal expansion coefficient and an improvement in mechanical strength through use of the refractory filler. Conventionally, lead titanate or the like having low expansion has been used as the refractory filler. However, as with the glass powder, the refractory filler is demanded to have a composition from which PbO has been removed. Accordingly, as the refractory filler, there has been considered use of willemite, cordierite, tin dioxide, β-eucryptite, mullite, silica, β-quartz solid solution, aluminum titanate, zircon, and the like. Of those, willemite has low expansion and satisfactory compatibility with the Bi2O3—B2O3-based glass (hardly devitrifies the Bi2O3—B2O3-based glass at the time of sealing), and hence has attracted attention (see Patent Literature 3 and Non Patent Literature 1).